1. Field of the Invention
This invention relates to casting molds and cores of the type wherein foundry sand is bound with a binder whose major part is a condensation-reactive compound having methylol groups in a molecule, and more particularly to a disintegration assistant for improving the disintegration characteristics of the molds and cores after casting is completed.
2. Description of the Prior Art
In connection with conventional production techniques for molds and cores used in casting, shell molding has been commonly used in which the molds and the cores are formed by binding foundry sand, for example, with a binder of phenolic resin regardless of the kind of alloys to be casted. Particularly, the shell molding has been frequently used for production of the cores beqause of superiority in productivity and dimensional accuracy.
However, in case the core produced by the shell molding is used in casting of a light alloy having a relatively low melting point such as aluminum alloy, a part of phenolic resin is subjected to thermal change under the heat of molten metal thereby to form very rigid graphite structure, so that the residual strength of the core after casting is considerably high. Accordingly, in order to facilitate disintegration of the core, the core is heated together with a resulting casting product at a high temperature such as about 500.degree. C. for a such long time as of 5 to 10 hours thereby to burn out the residue of the binder which has the graphite structure. This necessitates consumption of a large amount of energy. In this regard, it has been eagerly desired to develop a binder which is easily thermally decomposable to obtain molds and cores of high disintegration characteristics. From such view points, development of a variety of binders offering high disintegration characteristics to molds and cores has been extensively tried.
As a part of such development, investigation has been made on thermosetting resins containing no benzene ring, for example, unsaturated polyester and the like in view of the fact that formation of the graphite structure is due to the benzene ring of phenolic resin. However, such thermosetting resins are not sufficient in heat resistance as compared with phenolic resin and lower in hot strength. Furthermore, such thermosetting resins are too thermally decomposable, and accordingly gas defect is liable to arise when used for producing molds and cores, thereby lowering production yield of the molds and cores.
Moreover, from the view point of energy saving upon paying attention to the fact that heating is necessary to form molds and cores, i.e., to solidify the binder, studies have been made to obtain mold and core forming methods in which binders can be solidified at ordinary temperature. As one of these methods, so-called cold box method has been developed in which the combination of phenolic resin composition and isocyanate compound is used as the binder for foundry sand. However, phenolic resin is used also in this method, and therefore the disintegration characteristics of molds and cores after casting is inferior.